Two feeding trials were carried out to evaluate the influence
of by-pass protein on the intake and live-weight gain (LWG) of
growing cattle consuming a basal diet of fresh cut forage. A
third trial was carried out with rumen fistulated dry cows to
evaluate the effect of these supplements on some characteristics
of rumen fermentation. The use of fish meal as a source of
by-pass protein resulted in moderate increments in LWG without
effects on forage consumption when substituting urea in a
supplement based mainly on maize meal. The use of urea increased
the rumen ammonia N concentration but did not affect the
degradation of forage in nylon bags. The use of a supplement
based exclusively on sources of by- pass protein, such as fish
meal and dehydrated brewers grains resulted in small
non-significant (P>0.05) increases in LWG compared with equal
amounts of a supplement which is highly degradable in the rumen.

KEY WORDS: By-pass protein, forage, cattle, rumen
degradation

Introduction

Large responses in live-weight gain (LWG) and intake have been
observed when low quality fibrous feeds are supplemented with by-
pass protein (Preston and Leng 1987; Leng 1990). These resources
have been defined by Leng (1990) as those with a dry matter
digestibility (DMD) lower than 45% and a crude protein (CP)
content below 8%. Cattle production in Tropical America is mainly
sustained in pastures with DMD and CP values below those found in
temperate regions (Minson 1981) but in a range above the limits
defined by Leng. The information available on the use of by-pass
protein sources as supplements for this category of feeds is less
abundant.

The results of two feeding trials comparing the responses in
LWG and intake to supplements containing by-pass protein or
nitrogen (N) degradable sources in growing animals consuming a
basal diet of cut fresh forage are presented. Some
characteristics of rumen digestion were evaluated in a third
trial with rumen fistulated animals.

Materials and methods

The feeding trials (Experiments 1 & 2) and digestion trial
(Experiment 3) were carried out in Maracay, which has a range in
monthly average temperatures from 23.4 to 26.5 °C and a mean
rainfall of 989 mm during the last 10 years.

Experiments 1 & 2

Completely randomized designs were used to compare three
treatments in each trial, consisting of supplements containing
feed sources of high (H) and low (L) rumen degradable N or a
mixture of both (HL) in Experiment 1, and supplements containing
urea (U), fish meal (F) or urea and fish meal (UF) as sources of
N in Experim. 2 (Table 1).

Table 1:
Concentrate formulations (%)

Experiment
1

Experiment
2 & 3

H

L

HL

U

F

UF

Maize meal
by-product

73.3

-

36.7

88.8

77.0

72.8

Sesame meal

20.0

-

10.0

-

-

-

Urea

2.0

-

1.0

3.8

-

3.8

Fish meal

-

20.0

10.0

-

21.0

21.0

Dehydrated
brewers grains

-

78.5

39.3

-

-

-

Sulphur

0.2

-

0.1

0.4

-

0.4

Mineral
mixture

3.0

-

1.5

5.0

-

-

Salt

1.5

1.5

1.5

2.0

2.0

2.0

Eighteen animals, 14 Brahman x Holstein and 4 Brown Swiss,
were used in each trial. Equal numbers of each sex were included
in Experiment 1 and 12 males and 6 females were used in
Experiment 2. The animals were housed in partially covered
individual pens of 2 x 5 m with concrete floors. Two feeding
troughs were available, one for the forage and the other for the
supplements. Water was permanently available in plastic buckets
of 50 litres.

Fresh forage was offered daily between 07:30 and 09:30h
allowing approximately 15% refusals. The forage had variable but
similar amounts of elephant (Pennisetum purpureum), para (Brachiaria
mutica) and star (Cynodonplectostachyus)
grasses. The concentrate was offered between 07:00 and 08:00h,
1.5 kg/day in Experiment 1 and 2.0 kg/day in Experiment 2 during
the 84 days of duration of each trial.

Samples of forage offered and refused were taken daily, dried
for 48h at 65 °C and dry matter (DM) intake determined daily.
Subsamples were accumulated for four week periods to be analyzed
for N and ash (AOAC 1984), calcium (Fick et al 1979) and
phosphorous (Harris and Popat 1954). Samples of concentrate were
taken from each mixing batch and were also analyzed for those
fractions. In vitro organic matter digestibility (IVOMD,
Alexander and McGowan 1967) was determined in concentrate
samples. The animals were weighed weekly before the morning
feeding and the LWG estimated as the slope of the simple
regression of weights on time.

Experiment 3

A Latin square 3 x 3 was used with 17 day periods and three
rumen fistulated cows. The concentrates U, F and UF described in
Experiment 2 were compared fixing the proportion of concentrate
in each diet at 25% on a DM basis. The supplement to be offered
to each animal was calculated daily according to forage
consumption the day before. Feed and animal management was
similar to the first two trials. This experiment was conducted
simultaneously with Experiment 2 and the same forage was offered.

Rate of digestion of silage, maize meal and fish meal dry
matter were estimated using the nylon bag technique (Orskov et
al 1980) collecting two samples of each feed at 6, 12, 24, 48
and 72h during the last three days of each period. A sample of
the forage offered was oven dried at 65 °C for 48h, ground
through a 3 mm screen and 5 g introduced into each bag. The rate
of digestion was calculated as the half time (T2) using the
procedure described by Kempton (1980). Rumen liquid samples were
taken just before feeding, every hour after feeding for 6h and
every 2h during the following 12h of day 15 in each period and
filtered through cheese cloth. pH was measured and two samples of
30 ml acidified with eight drops of 97% sulphuric acid and stored
in a freezer. The samples were analyzed for volatile fatty acids
(VFA) and ammonia N following the methodology described by FAO
(1986).

The DMD of the diets was determined with 9 adult sheep, 3 per
treatment, with total collection of faeces. The animals received
the rations for 24 days and the measurements were carried out
during the last seven.

Results

The chemical composition of forages and feeds used is shown in
Table 2. Concentrates in Experiment 1 had a similar content of
CP, calcium and phosphorous, but the IVOMD of concentrate L was
almost 30 percentage units lower than concentrate H, and
concentrate HL was intermediate. CP content of concentrate UF in
Experiment 2 was as expected higher than the others, and
concentrate U was slightly higher than F. IVOMD was somewhat
lower in concentrates with fish meal.

The forage consumption in Experiment 1 was similar in all
treatments (Table 3) and LWG were slightly, though not
significantly (P>0.05), higher in treatments with by-pass
protein.

No differences in forage intake were observed between
treatments in Experiment 2, but LWG were higher (P<0.05) in
animals receiving concentrates with fish meal (Table 4). The
addition of urea to the concentrate with fish meal did not
increase LWG.

The results of Experiment 3 are shown in Table 5. The intake
per unit body weight was lower than in Experiment 2 and again no
differences were observed between treatments. The values of pH
were similar in all treatments with mean values per hour varying
from 6.5 to 7.0. Ammonia N concentrations per day were higher in
treatments with urea, reaching a peak of about 330 mg/litre two
hours after feeding against 170 mg/litre in treatment F. These
values decreased to about 80 mg/litre ten hours after feeding in
all treatments and remained at this level for the rest of the
day. Molar proportions of acetic, propionic and butyric acids
were similar in all treatments. The rate of digestion of forage
estimated by T2 was similar in the three treatments. The T2 of
maize meal was about 20h and for fish meal it was 113h. The DMD
of the diets determined with sheep were 63.3, 62.8 and 63.2% for
U, F and UF, respectively (P>0.05).

The LWG response to a supplement based exclusively on sources
with a high proportion of by-pass protein and low digestibility
was low when compared to a concentrate high in degradable N and
degradable organic matter (Table 3). A large proportion of the
concentrate L was dehydrated brewers grains (DBG, Table 2), which
has a low digestibility, decreasing the level of energy
supplementation compared with concentrate H. DBG also has a low
rate and extent of DM disappearance in the rumen (Parra et al
1984), decreasing the amount of organic matter available for
microbial protein synthesis. Both effects tended to out weigh the
benefits of this supplement as a source of by-pass protein and
resulted in the small increases in LWG which were obtained.

The addition of fish meal as a source of by-pass protein to a
concentrate based on maize meal resulted in moderate and
significant responses in LWG compared with urea as a source of
degradable N, but without effects on forage consumption. Similar
results have been obtained in other trials carried out in
Tropical America using forages and silages as the base of the
diets (Combellas 1991). Higher responses in LWG and increases in
roughage consumption have been observed when by-pass protein is
supplemented to diets poor in N, but this was not the case
because the control diet (Treatment U) was supplemented with a
concentrate high in degradable N (Table 2). The addition of urea
to the concentrate with fish meal (Treatment UF) had no effects
on animal response (Table 4) indicating that the small proportion
of degradable N of fish meal complemented other sources of this
nutrient in the diet and was enough to fulfil the requirements of
microorganisms. The urea in treatments U and UF increased the
level of ammonia N in the rumen (Experiment 3, Table 4), but did
not improve the rate of digestion of forage in nylon bags.
Probably, as was suggested by Leng (1990) for low quality fibrous
diets, the concentration of ammonia N required in this type of
diet to optimize intake and forage utilization probably decreases
when by-pass protein is offered.

The results of Experiment 2 have shown, as was observed in
previous trials (Combellas 1991), that the supplementation with
by-pass protein in diets based in tropical forages and silages
with adequate amounts of degradable N result in moderate
increases in LWG without raising forage consumption. However, the
substitution of high rumen degradable supplements by sources of
by-pass protein of low digestibility (Experiment 1) decreases the
LWG responses in growing cattle.

Acknowledgements

This research was partially sponsored by the International
Atomic Energy Agency, Project N? VEN/5/013.

References

Alexander R H and McGowan M 1966 The
routine determination of invitro digestibility of
organic matter in forages, an investigation of the problems
associated with continuous large scale operation. Journal of the
British Grassland Society 21:140-147